Enhanced Phosphate Consumption Stimulated by Nitrogen Fixation Within a Cyclonic Eddy in the Northwest Pacific

Mesoscale eddies are common in the subtropical Northwest Pacific, however, relatively little is known about their spatial variability and temporal evolution, and how these impact upper ocean biogeochemistry. Here we investigate these using observations of a cyclonic eddy carried out along four sequential transects. Consistent with previous observations of cyclonic eddies, the eddy core had doming isopycnals, bringing elevated nutrient waters nearer to the surface. However, we also found that the upper layer of the eddy above the nutricline had significantly lower phosphate concentrations within its core relative to its edge. We attributed this to elevated N-2 fixation within the eddy core, which was likely driven by enhanced subsurface iron supply, ultimately resulting in increased phosphate consumption. Eddy-enhanced N-2 fixation was additionally supported by the elevation of nitrate + nitrite to phosphate ratios below the euphotic zone. Moreover, we observed that while the upward displacement of isopycnals within the eddy core led to an increase in phytoplankton biomass in the lower euphotic zone, there was no significant increase in total phytoplankton biomass across the entire euphotic zone. Cyclonic eddies in the subtropical North Pacific are projected to be becoming more frequent, implying that such dynamics could become increasingly important for regulating nutrient biogeochemistry and ultimately productivity of the region.

Automated summary

This study investigates the spatial variability, temporal evolution, and impact of mesoscale eddies on upper ocean biogeochemistry in the subtropical Northwest Pacific. The results show that the upper layer of the eddy has lower phosphate concentrations within its core, which can be attributed to elevated N-2 fixation driven by enhanced subsurface iron supply. The presence of eddies affects the phytoplankton biomass in the lower euphotic zone but does not significantly impact the total phytoplankton biomass across the entire euphotic zone. With an anticipated increase in the frequency of cyclonic eddies in the subtropical North Pacific, their role in regulating nutrient biogeochemistry and productivity of the region may become more important.